Round Up or Round Down

When instructing workshops for electricians, I have found that some of the participants get very confused about how the National Electrical Code (NEC) addresses the rounding up or rounding down of the size of an overcurrent protection device (OCPD) to allow a motor to start and accelerate its driven load.

Round down or round-down size

To demonstrate the selection of the OCPD by rounding down, let’s look at a 50 horsepower (hp), 460-volt, three-phase, motor with a Design B classification. Section 430.6(A)(1) requires that the full load current (FLC) in amps of Table 430.250 be used to determine the components needed to complete the branch circuit supplying power to the motor. The percentage found in Table 430.52, based on the type OCPD, is chosen, and the FLC in amps is multiplied by this value.

If a circuit breaker (CB) is used, the FLC of 65 amps is multiplied by 250 percent, which derives 162.5 amps (65A × 250 percent = 162.5A). By the requirements of 430.52(C)(1), the OCPD must not exceed 250 percent. Therefore, the size of the CB, when rounded down is 150 amps per 240.6(A). If this device size is not adequate to allow the motor to start and run, the CB size can be rounded up to 175 amps as permitted in Ex. 1 to 430.52(C)(1).

Should the motor still trip open the CB when trying to start and drive the load, the Ex. 2(c) to 430.52(C)(1) can be applied at 400 percent (65A × 400 percent = 260A) to permit a 250-amp CB to be used. As the exception points out, when this rule is applied, the round-up size above 400 percent is not permitted.

Sizing conductors to supply motor

The conductors are sized by the provisions of 430.22(A) and are not required to be increased to pair up with and match the size of the OCPD.

For example, the motor’s FLC of 65 amps is multiplied by 125 percent (65A × 125 percent = 81.25A) that is equal to 81.25 amps. Say the CB and motor terminals are rated at 75°C as outlined in 110.14(C)(1). Then, if THWN insulation per Table 310.10 is selected, the size conductors would be 4 AWG copper. By increasing the size of the conductors by 125 percent, the conductors and windings of the motor are protected from overload conditions until the overload device has time to trip.

Remember, Table 310.13 pertains mainly to the conductor’s conditions of use while the ampacities of conductors are found in Table 310.16. Also note that conductors supplying a motor are not required to be increased to the OCPD that is sized to allow the motor to start and run.

Sizing controllers

Controllers are sized according to the provisions of 430.83(A)(1), which requires the controller to be equivalent to the hp of the motor. In selection, the controller for our 50 hp motor is rated 50 hp or greater. Naturally, the controller can be an individual controller or one placed in a motor control center (MCC).

Sizing overloads

Fuses sized at 125 percent of the FLC of the motor’s nameplate in amps can be used to provide overload (OL) or backup protection for the motor’s windings and other circuit components.

Section 430.6(A)(2) states that the nameplate amps are used for determining overload protection (OLP). For example, a motor with a nameplate value of 62 amps, the size fuses for OLP is approximately 70 amps (62A × 125 percent = 77.5A) when rounded down. However, be careful not to exceed the 125-percent rule per 430.32(A)(1), when fuses are used as the OLP device. Remember, the nameplate of 62 amps is used for selecting the size overload relays to be installed in an individual controller or MCC. If selected properly, the overloads will protect the motor and its associated components.

Using the disconnecting means as a nonfusible or fusible type, review sections 430.57, 430.55 and 430.32(A)(1). High (motor) starting currents may necessitate larger OL relay units. When this occurs, carefully review 430.32(C).

Control conductors and protection

Control conductor and overcurrent protection sizes are located in Table 430.72(B) and are based on the conductors either remaining or leaving the enclosure housing the controller’s operating components. Master and emergency stop buttons can be used to interrupt the power to the coil of the controller and disconnecting the power supply to the motor. This automatic disconnection is usually a safer method than manually disconnecting with a safety switch. Note that the disconnecting means for the controller and motor can be sized per 430.110(A) and 430.109(A)(1).

Hopefully, this article explains the round-down or round-up method for sizing and OCPD for a motor as well as procedures for sizing and selecting other related motor circuit components. For more information, review the articles of the NEC mentioned herein.

STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.